# Four Forces vs. Fifth Force: Physicist's Insight

• chaszz
In summary, the physicist says that there are only four forces and that for the first time in his life, it struck him as incomplete. He is asking if the force which is causing the universe to expand and which is speeding up, not at least a candidate for a fifth force, can be ruled out. He says that the force depends upon pressure and that with normal matter, which usually has negligible pressure compared to its mass energy, gravity behaves as we would naively expect. However, with radiation, if you have a box full of radiation, then that radiation will exert a pressure on the walls of the box equal to one third its energy density. He says that with dark energy, which has negative pressure, the opposite effect happens:f

#### chaszz

I read an article the other day by a physicist who said there are only four forces and for the first time in my life it struck me as incomplete. Is not the force which is causing the universe to expand and which is speeding up, not at least a candidate for a fifth force? Since we know so little about it, can it be ruled out?

I read an article the other day by a physicist who said there are only four forces and for the first time in my life it struck me as incomplete. Is not the force which is causing the universe to expand and which is speeding up, not at least a candidate for a fifth force?
It may be force but there's no reason yet to think it is a fundamental force.

Since we know so little about it, can it be ruled out?
The question isn't can it be ruled out; the question is: is there sufficient cause to rule it in. i.e we don't invent new entities until the old ones are proven inadequate.

I always thought that quantum pressure (the effect that prevents two odd-half-spin particles from occupying the same location if they are in the same quantum state) should be called a fifth force, or at least a fourth force if electroweak is a single force. Quantum pressure prevents certain high-density stars from collapsing into black holes. However, it can't be called the "cause" of expansion, unless some very circuitous reasoning is applied.

This 'cause' is supposedly said to be dark energy. And as we all know, we are still in the dark about it.

Sigh.

Anyways, what makes a particular form of interaction eligible to be called a fundamental force? Should it be unexplainable using conventional theories and the other 4 forces?

I read an article the other day by a physicist who said there are only four forces and for the first time in my life it struck me as incomplete. Is not the force which is causing the universe to expand and which is speeding up, not at least a candidate for a fifth force? Since we know so little about it, can it be ruled out?
You don't need a fifth force for this. It's just gravity.

There are a number of extremely careful and detailed searches for additional forces. So far they all have come up empty.

You don't need a fifth force for this. It's just gravity.

Since it appears to do the opposite of what gravity does, how can it be gravity? Not scoffing, just asking. What steps of empirical observation, therorizing, conception, reasoning, math, or whatever could be conceivably be used to demonstrate that it is gravity? Again, I am not scoffing, but sincerely interested in the reply.

Since it appears to do the opposite of what gravity does, how can it be gravity?
In General Relativity, the gravitational force depends not only upon energy density (in Newtonian mechanics, this is just the mass), but also upon pressure. With normal matter, which usually has negligible pressure compared to its mass energy, General Relativity describes gravity as we expect it to behave. If instead you have matter that has significant positive pressure, such as electromagnetic radiation (light), then it tends to gravitate and respond to gravity more strongly than we would naively expect (this is observed by the deflection of light waves by gravitational lensing being twice what we would expect from Newtonian gravity).

But dark energy has negative pressure. A very large negative pressure. So it has the opposite effect: gravity actually causes dark energy to want to push away from itself. This causes the accelerated expansion.

Chalnoth, what pressure do you mean? Are you sayng that a mass which emits a lot of EM radiation experiences a stronger gravitational attraction to another mass?

How does this lead to negative gravitational force and a possible explanation for dark energy expansion? (which is itself an increase in the measurement of distance between two very distant gravitationally unbound bodies)

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Chalnoth, what pressure do you mean?
Well, with radiation, if you have a box full of radiation (this happens naturally if you just heat the box up), then that radiation will exert a pressure on the walls of the box equal to one third its energy density.

By contrast, if you fill a box with gas, though it will exert some pressure, that pressure will be completely and utterly negligible compared to its energy density (because the energy density includes its mass energy).

Are you sayng that a mass which emits a lot of EM radiation experiences a stronger gravitational attraction to another mass?
No. I'm saying that the radiation itself exerts a gravitational force. This is particularly important in the very early universe, when most of the energy density was in radiation. At that time, when our universe was dominated by radiation, the extra gravitational attraction due to the pressure of the radiation caused the expansion to slow even more rapidly than it did later, when the dominant energy was in matter.

Individual objects, by contrast, can never emit enough radiation to compete with their mass energy, just because mass energy is so tremendously large.

How does this lead to negative gravitational force and a possible explanation for dark energy expansion? (which is itself an increase in the measurement of distance between two very distant gravitationally unbound bodies)
Well, you can calculate it explicitly in General Relativity. Positive pressure = stronger attraction, expansion slows down faster. Negative pressure = weaker attraction, potentially even repulsion, causing the expansion to either slow down more slowly or speed up.

No. I'm saying that the radiation itself exerts a gravitational force.

Thanks Chalnoth, I didnt know this. Does this mean that radiation (photons) has mass? Is this the same mass pressure that would make a solar sail work?

I still can't see how you could ever get negative pressure or negative gravity from this.

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Thanks Chalnoth, I didnt know this. Does this mean that radiation (photons) has mass? Is this the same mass pressure that would make a solar sail work?
No, not at all. In General Relativity, gravity doesn't respond to mass. It responds to energy density, momentum density, pressure, and shear (twisting forces).

That energy density includes mass energy, which completely dominates all of the other components for normal matter (except in extreme conditions, such as neutron stars).

I still can't see how you could ever get negative pressure or negative gravity from this.
Well, some forms of matter are weird. The cosmological constant can be seen as a sort of matter with pressure that is equal to its energy density, only negative. A large number of quantum mechanical fields that are relatively easy to write down on paper (though we have no evidence for) also have similar negative pressure.

Is there a short explanation of negative pressure somewhere? Otherwise I'll write one here. (I.e., photons exert pos. pressure so as a piston expands, they lose energy, and the total # of photons remains same. In contrast, dark energy, if it is other than just a Λ term, exerts neg. pressure, so energy per particle increases w/ expansion, required to keep dark energy density a constant.)

Chalnoth is then an accepted explanation for dark energy? I thought it was not possible at present to explain dark energy.

Chalnoth is then an accepted explanation for dark energy?
No, Chalnoth is then an accepted explanation for loud, vulgar eructations and noxious fumes but he is not an explanation for dark energy. Chalnoth is then an accepted explanation for dark energy? I thought it was not possible at present to explain dark energy.
Well, there are many possible explanations. The simplest explanation that agrees with the current data is the cosmological constant, and as a result it is the leading explanation. But so far the evidence is far, far too sparse for anybody to say with any degree of confidence that the cosmological constant is the explanation.

So it's not so much that we can't explain it, but rather the problem is that the evidence is far too sparse at the moment to say which of the many possible ideas is the correct one (if any).

Dave lol! I forgot to add "this"

One of these days I am going to have to find out what GR is really all about. I did SR and QM (but apparently no one understands that anyways). The past 30 years I have been involved with RF so I forgot most of what I knew about the rest of Physics.

Is there a short explanation of negative pressure somewhere? Otherwise I'll write one here. (I.e., photons exert pos. pressure so as a piston expands, they lose energy, and the total # of photons remains same. In contrast, dark energy, if it is other than just a Λ term, exerts neg. pressure, so energy per particle increases w/ expansion, required to keep dark energy density a constant.)

This page has a nice explanation of why the vacuum has negative pressure. The mechanism is easily understood, it is the magnitude that we have difficullty explaining.

http://www.astro.ucla.edu/~wright/cosmo_constant.html

In the piston analogy I thought it was the pressure on the outside from randomly moving air molecules pushing the piston back in?

In the piston analogy I thought it was the pressure on the outside from randomly moving air molecules pushing the piston back in?
In this analogy, consider the piston to be in a vacuum.

In the piston analogy I thought it was the pressure on the outside from randomly moving air molecules pushing the piston back in?

Normally you have a gas inside the chamber and nothing outside. The moving gas molecules colliding with the piston push it outward. This is positive pressure. To understand the negative pressure of the vacuum, imagine that you have vacuum inside the chamber and 'nothing' outside, and further imagine that the vacuum has some non-zero energy density. As the piston moves outward, you have to pull on it in order to create new vacuum, since the energy to create the new vacuum has to come from somewhere. So there is a force pulling the piston inward. This is negative pressure. When you insert this into Einstein's field equations, it leads to a repulsive term identical to the cosmological constant. Here's another website with some explanations.

http://ned.ipac.caltech.edu/level5/Guth/Guth_contents.html

phyzguy I thought we trying to explain the source of the force driving expansion? negative gravity etc? I am reading this negative pressure as working in the same direction as gravity.

Well, there are many possible explanations. The simplest explanation that agrees with the current data is the cosmological constant, and as a result it is the leading explanation. But so far the evidence is far, far too sparse for anybody to say with any degree of confidence that the cosmological constant is the explanation.

So it's not so much that we can't explain it, but rather the problem is that the evidence is far too sparse at the moment to say which of the many possible ideas is the correct one (if any).

Can you name other possible explanations, please?

phyzguy I thought we trying to explain the source of the force driving expansion? negative gravity etc? I am reading this negative pressure as working in the same direction as gravity.
I agree it seems counter-intuitive. One would think that a negative pressure (i.e a tension pulling inwards) would lead to a contraction, not an expansion. All I can say is that when you analyze Einstein's field equations it leads to an expansion - i.e. your intuition is wrong, and we have to trust the mathematics. Perhaps someone else has a better explanation.

I agree it seems counter-intuitive. One would think that a negative pressure (i.e a tension pulling inwards) would lead to a contraction, not an expansion. All I can say is that when you analyze Einstein's field equations it leads to an expansion - i.e. your intuition is wrong, and we have to trust the mathematics. Perhaps someone else has a better explanation.

I was under the impression that Einstein "fudged" his equations to insert the cosmological constant after he realized the universe would collapse without it, as he and everyone else at the time thought the universe was static. When Hubble later found the universe was expanding, Einstein called that "fudge" factor the biggest blunder of his life. So are we dealing with math that could be altered to work either way? And, has any experimental proof come along since then to demonstrate that the second version of the equations is more accurate than the first? Which set is recognized as the defining version of GR?

How can the universe ever collapse if the universe is infinite and the force is everywhere and in every direction?

Can you name other possible explanations, please?
Well, most of the other explanations go under the name "quintessence", and are described by some sort of quantum scalar field with a particular potential. Wikipedia has a very rough description:
http://en.wikipedia.org/wiki/Quintessence_(physics [Broken])

The main thing to take away here is that none of these models are very well-motivated. They are all very much ad-hoc. The best we've got, really, is the cosmological constant.

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I was under the impression that Einstein "fudged" his equations to insert the cosmological constant after he realized the universe would collapse without it, as he and everyone else at the time thought the universe was static. When Hubble later found the universe was expanding, Einstein called that "fudge" factor the biggest blunder of his life. So are we dealing with math that could be altered to work either way? And, has any experimental proof come along since then to demonstrate that the second version of the equations is more accurate than the first? Which set is recognized as the defining version of GR?
Basically, if you look at the derivation of Einstein's equations, there is no good reason to leave the constant term out. The reason why it has been left out for so long is because simple back-of-the-envelope calculations demonstrated that this term had to be incredibly small, around $10^{-120}$ in natural units, in order for our universe to exist at all. So people figured it must actually be set to zero by some symmetry or other. But nobody has ever found a symmetry that sets the cosmological constant to zero.

But now we seem to be measuring it, at right around $10^{-120}$.

As for Einstein's blunder, it really was a tremendous blunder. It was a blunder for two big reasons:

1. His introduction of the cosmological constant was done for the exact wrong reason. He had a preexisting prejudice that our universe should be static, and so he added the factor to make it static. This is just the wrong way to do science, and it was an absolute mistake to rely on this prejudice.
2. Even worse, however, was the fact that a universe whose expansion was balanced by a cosmological constant is unstable. Move a single galaxy a little bit out of place, and it will cause the place it moves to to start collapsing, the place it moves from to start expanding. So in actual fact a universe whose expansion is balanced by a cosmological constant doesn't work: it's like a pencil balanced on its tip: it's guaranteed to fall at some point, no matter how carefully you balance it.

I recall reading many years ago (>30) that we didnt know whether the universe had sufficient mass so that gravity would slow the rate of expansion until eventually the universe begin to collapse. ie a big bang in reverse. I realize that this is way out of date, but I wanted to ask how, with this simplistic model, could the universe collapse since the universe is infinite, and the pull of of gravity is the same in all directions and everywhere?

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I recall reading many years ago (>30) that we didnt know whether the universe had sufficient mass so that gravity would slow the rate of expansion until eventually the universe began to collapse. ie a big bang in reverse. I realize that this is way out of date, but I wanted to ask how, with this simplistic model, could the universe collapse since the universe is infinite, and the pull of of gravity is the same in all directions and everywhere?
Yes, if there was enough mass relative to the current rate of expansion and no dark energy, or if the dark energy at some point in the future goes away and it turns out that the remaining mass is large enough compared to the rate of expansion, then it would recollapse. And in General Relativity, it would collapse to a singularity. Though to be fair, General Relativity cannot be trusted that far.

Is it correct that if it wasnt for the recent increase in dark energy expansion, that the matter density would eventually become higher in the filaments between voids and the sizes of galaxies would increase due to filament collapse and galaxy collisions? If so would this constitute another happy "goldilocks" coincidence?

Yes, if there was enough mass relative to the current rate of expansion and no dark energy, or if the dark energy at some point in the future goes away and it turns out that the remaining mass is large enough compared to the rate of expansion, then it would recollapse. And in General Relativity, it would collapse to a singularity. Though to be fair, General Relativity cannot be trusted that far.

Chalnoth In this simple model I can't see what causes the collapse since the universe is infinite and the forces are approximately equal in all directions. Unless the small differences in gravity caused instability and accumulaton of matter in certain locations?

Is it correct that if it wasnt for the recent increase in dark energy expansion, that the matter density would eventually become higher in the filaments between voids and the sizes of galaxies would increase due to filament collapse and galaxy collisions? If so would this constitute another happy "goldilocks" coincidence?
My impression is no, not if the universe didn't recollapse. My understanding of structure formation (admittedly basic understanding) is that in an expanding universe, matter that is within the Jeans' length collapses in on itself, while matter outside that length does not. So basically, structures will gobble up all the local matter they can, then stop growing. Dark energy turns off this growth sooner, but I think it stops in any expanding universe.

This isn't the case for a universe that recollapses, because the collapse brings matter closer together, causing more things to enter the Jeans' Length.

But admittedly I could be mistaken. I don't have experience with the non-linear growth of structure.

Chalnoth In this simple model I can't see what causes the collapse since the universe is infinite and the forces are approximately equal in all directions. Unless the small differences in gravity caused instability and accumulaton of matter in certain locations?
Think of an expanding universe like a fluid. If this fluid is made out of normal matter, then the gravity between the particles of the fluid wants to pull the whole thing together. So if it is expanding, gravity acts to slow the expansion by pulling everything closer to everything else. If this pull is strong enough, by the density being large compared to the expansion rate, then it will cause the universe to recollapse in on itself.

Chalnoth thanks for the Jeans length reference, I think this answers questions above that I have had for quite a while. Although, what is the estimated value, it seems like it isn't that large? Wouldnt this be relevant for star formation?

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